1. Field of the Invention
This method relates to a temperature controlled refresh feature such as the feature specified for DDR4 memories, and more particularly, to a temperature controlled refresh feature which uses a counter in order to track external refresh commands, and selectively carry out or skip an internal refresh according to the counter value.
2. Description of the Prior Art
DDR4 is the next generation of DRAM memories, promising faster operating speeds than DDR3, as well as greater power saving and reduction features. The standard operating voltage has been reduced from 1.5V for DDR3 to 1.2V for DDR4, which makes DDR4 suitable for many mobile and handheld devices that require greater power efficiency.
Another feature of DDR4 which promises power reduction is Temperature Controlled Refresh. External refresh commands are usually set so that the DRAM is refreshed every 7.8 micro-seconds, but the length of time for which data can be retained by a DRAM is greater at lower operating temperatures. By monitoring the operating temperature of the DDR4 with a built-in temperature sensor, external refresh commands can be skipped when the DDR4 is at lower temperatures, and this feature can then be de-activated as the temperature rises to normal operating conditions. This can give up to a 20% reduction in power.
The current specification for DDR4 DRAM specifies a normal mode, where external refresh commands are provided every 7.8 micro-seconds, and an extended temperature mode, where external refresh commands are provided every 3.9 micro-seconds. The mode is selected using MRS commands. The DDR4 spec provides three temperature ranges, which are: less than 45° C., 45° C. to less than 85° C., and 85° C. to less than 95° C. If the DRAM is to be operated at 85° C. and above, it must be programmed to the extended temperature mode and the DRAM controller must provide refresh commands every 3.9 micro-seconds according to the extended temperature mode of operation. By using the Temperature Controlled Refresh function, certain refresh commands can be skipped to conserve power while still guaranteeing that data will be safely maintained. The Temperature Controlled Refresh function is activated using MRS commands. Once activated, the DRAM will manage this feature automatically for either the normal or extended temperature mode. In certain modes and in a certain temperature range, half of the external refresh commands may be skipped. In other conditions, different ratios of external commands to internal refresh could be provided making further power savings achievable.